The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the second law declares the impossibility of machines that generate usable energy from the abundant internal energy of nature by processes called perpetual motion of the second kind.
The second law of thermodynamics applies in many specific ways, for example, in that any system involving measurable heat transfer has some irreversible energy loss to heat. Although there have been many experiments to prove exceptions to this general rule, no device yet exists that harnesses thermal energy and converts it into electricity without loss.